The invention relates to the soldering of components onto printed circuit boards.
Increasing use is being made in recent times of so called surface mounted components which generally have small dimensions and which are soldered onto a printed circuit board by means of a reflow soldering process.
On a large number of printed circuit boards there are however components present for which the reflow process is less suitable. These are generally components such as connectors and electrolytic condensers which are provided with conventional connecting wires and wherein, in order to ensure a fixing and electrical connection of the component to the printed circuit board, the connecting wires extend through openings arranged in the printed circuit board to below the underside of the circuit board. On the underside, a soldered connection must then be made to the tracks present on the underside of the circuit board.
Particularly when a printed circuit board is provided with components on both sides, it is not always possible to make use of a classical wave soldering process for this purpose; this could after all unsolder or destroy the components previously mounted by means of a reflow process.
So as to avoid these problems, a soldering apparatus is known, comprising:
a vessel for containing and heating molten solder;
transporting means for transporting objects for soldering over the vessel;
at least one tube extending substantially vertically above the soldering vessel;
pump means for feeding molten solder to the underside of the tube; and
moving means for moving into at least the vicinity of each other the underside of the objects for soldering and the upper side of the tube.
The tube thus forms a kind of small wave soldering apparatus inside which one or more connecting wires can be soldered simultaneously. It is herein possible to carry out the soldering process only at the locations where this is desired, i.e., at the locations where for instance the connecting wires of connectors or electrolytic condensors or other large components extend through openings arranged in the printed circuit board to below the underside of the circuit board. The remaining parts of the underside of the printed circuit board are herein not exposed to solder, so that the components present there are not removed or destroyed.
This prior art apparatus nevertheless has one drawback, this being that, in order to ensure a sufficiently high temperature at the top part of the tube so that a good soldered connection is obtained, a flow of molten solder must be maintained through the tube. The solder herein serves as carrier of the heat necessary for the soldering process and as constant source for feeding solder to the connection.
According to the prior art a smaller or larger recess is herein made in the upper edge of the tube. A flow of solder can hereby be maintained even when the top part of the tube is moved into the vicinity of the underside of the printed circuit board or against it. A quantity of solder herein exits laterally at the position of the smaller or larger recess. This means that in the design of the printed circuit board space must be allowed for this outflowing stream of solder; no components may after all be present at this location. The soldering apparatus can consist of one or more tubes. The tube cross-section is adapted if necessary to the parts for soldering.
The object of the present invention is to provide such a soldering apparatus wherein these drawbacks are obviated.
This objective is achieved in that an outflow opening is arranged in each of the tubes at some distance from the upper side of said tubes.
By arranging this outflow opening at some distance from the upper side of the tube the outflowing stream of solder will no longer contact the printed circuit board for soldering and the components present thereon, thus allowing more freedom in the design of the printed circuit board and the placing of the components thereon.
It is pointed out here that the heat transfer to the solder on the upper side of the tube takes place through heat flow inside the solder resulting from the thermal contact with the solder flowing thereunder out of the outflow opening. Experience has shown that, with a careful choice of the distance in question, the fact that the solder does not flow over the upper edge of the tube does not have to be a drawback, and that it is hereby possible to obtain a soldered connection of good quality. For the moment a distance is assumed of 10-18 mm, preferably 12-15 mm. This assumes a tube height of about 40 mm.
According to a first preferred embodiment the moving means are adapted to cause the upper side of the tube to contact the underside of the printed circuit boards for soldering.
This embodiment is particularly suitable for tube materials which result in a concave or flat meniscus of the solder, wherein the pressure generated by the solder pump pushes the solder against the printed circuit board. In contrast hereto, it is also possible to place the upper side of the tube a short distance from the printed circuit board; this is intended for situations where the solder has a convex meniscus.
Although the invention can be applied in a situation where only one component has to be selectively soldered, wherein only one tube is necessary, it is also possible that a larger number of tubes is required because a larger number of separate components must be soldered.
In such a situation it is attractive if each of the tubes is fixed with its underside to the same substantially horizontal plate. This situation will result in a considerable simplification of the, moving means for moving each of the tubes in substantially vertical direction, or during the movement of the soldering side of the printed circuit board wherein dimensional tolerances in the product for soldering are otherwise difficult to overcome.
According to another preferred embodiment a number of spacers are present on the plate.
The position of the circuit board in relation to the upper sides of the tubes can hereby be determined precisely. Use is in any case usually made of robots to place the printed circuit board at the soldering position.
A more specific embodiment teaches that the height of the spacers is adjustable.
It is hereby possible to precisely adjust the distance between the upper side of the tube and the printed circuit board. Although it is attractive in most cases to have these make contact, it is better in some cases, depending on material of the tube and solder type, to leave a small distance clear between the upper side of the tube and the underside of the circuit board.
In order to obtain an optimal soldering temperature it is attractive if the pump means are adapted to temporarily increase the pump capacity during the soldering operation.
The solder flow rate is hereby increased temporarily, so that at the top of the tube, where the soldering process must take place, xe2x80x9cfreshxe2x80x9d, non-oxidized solder coming from the soldering vessel is available which has the correct temperature.
According to another preferred embodiment the pump means are adapted to temporarily increase the pump capacity prior to soldering to a value which is higher than the value during soldering.
This embodiment shortens the process of xe2x80x9cflushingxe2x80x9d the solder in the solder tower and preheating thereof.
It is noted here that the pump capacity at rest is relatively low. The solder level in the tube hereby remains below the lateral outflow opening, whereby solder will no longer flow out of the tube and fewer solder oxides will thus be formed.
According to a specific preferred embodiment the size of the outflow openings is adjustable.
During performing of the soldering process the flow rate out through the outflow opening is a very decisive factor in the constant level of the solder in the tubes. Particularly in the case of types of component with which little experience has yet been acquired, it is attractive to be able to set as many variables as possible. The size of the outflow opening is one of these variables so that it is good to be able to adjust the size of this opening.
According to a further embodiment a sleeve is arranged around or inside at least one of the tubes with an adjustable outflow opening, which sleeve extends at least over the outflow opening, wherein the sleeve is provided with an opening and wherein the sleeve is movable between a position in which the opening in the sleeve coincides with the outflow opening and a position in which the opening in the sleeve at least partially covers the outflow opening.
This represents an attractive embodiment in the situation with an adjustable outflow opening.
According to a further preferred embodiment the upper side of the tubes is covered with a grid. It is the intention with this construction to also reduce the formation of whiskers and bridges. A grid is not only understood to mean a structure of intersecting elongate elements, but also for instance a honeycomb structure or a structure of parallel extending elongate elements.
According to another preferred embodiment the plate is divided into separate elements which are individually exchangeable.
This construction results in a simplification of logistics; when the location of a component or group of components on a printed circuit board is changed, only a part of the whole plate with the tubes arranged thereon has to be replaced.
According to a specific embodiment hereof the elements of the plate are placed on a frame. This construction results in a greater rigidity of the base on which the tubes are placed, so that sagging of the plate and the associated drawbacks are avoided.
In this way a plurality of base plates with tubes can be mounted on the frame, so that a plurality of separate embodiments of a printed circuit board can be soldered without base plates having to be exchanged for the purpose.
It is the intention to carry out the soldering process with a minimum of flux. It is then essential or at least attractive to expel the oxygen from the environment of the soldering process.
According to an embodiment adapted for this purpose, supply means for low-oxygen gas such as nitrogen are arranged in the vicinity of the soldering vessel.
The ability to work without further xe2x80x9cconfinementxe2x80x9d of the low-oxygen gas requires a high flow rate of low oxygen gas, which has a cost-increasing effect and aerodynamic effects have an adverse effect on the action of the low-oxygen gas.
In order to prevent this, a cover plate is arranged according to a specific preferred embodiment above the supply means, which cover plate is removable when a soldering operation is performed. This plate can for instance be pushed away to the side.
It is however also possible that the cover plate be provided with apertures and that the cover plate be vertically movable to a position below the level of the upper part of the tubes. Both solutions have the drawback that the movement of the plate results in the escape of low-oxygen gas to the environment.
An attractive further embodiment therefore provides the measure that around the supply means there is arranged a screen, the upper edge of which corresponds with the contour of the printed circuit board for soldering. The movement of the cover plate is hereby dispensed with, while a kind of confining box is formed by the screen, the solder bath and the printed circuit board.
According to yet another embodiment the upper edge of the screen is manufactured from flexible material. A good fitting to the printed circuit board is hereby obtained so that the least possible low-oxygen gas leaks away.
Gas and solder are added to the space inside the box. The excess volume is discharged via gaps formed, according to a preferred embodiment, between the screen and the solder bath.
Other attractive preferred embodiments are stated in the remaining sub-claims.